My wife uses a 3gs, still on 4.0 firmware. This week she received a message from our provider, Softbank Japan, saying that due to some changes at their end which will happen in October, she may get some issues using the phone unless she upgrades to at least 4.2.
Now I don't want to upgrade as she will be due for a replacement soon, iPhone 5 or whatever, my plan is to jailbreak, unlock and sell it...
Anyone had a similar experience with other providers?

Same here, ps before you go about updating it I STRONY SUGGEST you ask for our help that way you don't accidentally screwup the baseband and possibly brick it from ever being unlockable or even jailbreakable